The present invention relates generally to inlet structures for aircraft engines, and more particularly, to an engine air inlet having a control mechanism to regulate airflow into the engine in accordance with low or high speed flight characteristics.
The primary purpose of an aircraft engine air inlet is to supply the correct quantity and quality of air to the compressor of the engine. The performance of the inlet is related to the following four characteristics: (1) total pressure recovery; (2) quality of air flow, i.e, distortion and turbulence; (3) drag; and (4) weight and cost. The performance of the inlet must be determined by simultaneously evaluating all four characteristics since the gain in one is often achieved at the expense of another.
Total pressure recovery of the inlet is an important measure of the inlet performance and is defined as the ratio of the total pressure at the compressor face to that of the free stream air flow. It is desired to recover as much of the total pressure at the compressor face as possible because the total pressure of the free stream represents the available mechanical energy of the flow which can be converted into a static pressure increase as the flow is decelerated through the engine. A large static pressure is desirable at the compressor face because the compressor section of the turbine engine does not have to be as large in order to compress the flow to the required pressure for combustion.
Another characteristic of inlet performance is the quality of the airflow delivered to the engine compressor. It is important that the distortion and turbulence of the flow at the compressor face be minimal, otherwise compressor stall or even flame out can result.
Although high performance is desired from the inlet, it must be balanced by tolerable weight and costs. While variable geometry inlets may improve performance over a range of operating conditions, it should be determined whether the improved performance is worth the added complexity, weight, and cost of the variable inlet geometry.
The inlet should be sized to provide the proper amount of air to the engine at all flight conditions. All inlet designs require compromises in order to achieve an acceptable performance throughout the variations in flight Mach number, angles of attack, and sideslip, as well as variations in the properties of the atmosphere. Normally, the most critical flight conditions or parameters are selected and the inlet is sized for these design conditions. Additional structures or methods are provided to optimize performance on so called "off-design" flight conditions.
At low speeds, for example, most inlets do not have enough cowl capture area to provide the required engine airflow. Cowl capture area is the cross sectional area of the inlet. Auxiliary doors or suck-in doors can provide additional air during take-off.
At higher speeds, however, excess air may exist in the inlet that must be dumped overboard. The excess air can either spill around the cowl lips or the excess air can be dumped through bypass doors. The inlet designer must also consider that both spillage and bypass air produce drag.
In light of the foregoing, there exists a need for a simplified, variable geometry, engine air inlet having a control mechanism that can regulate the airflow into the engine in accordance with low or high speed flight characteristics, especially "off design" flight conditions.